1. Field of the Invention
The present invention relates to a knife drive mechanism of a sewing machine, and more particularly to a knife mechanism having an upper knife which may be selectively connected with or released from its drive means.
2. Description of the Prior Art
In an overlocking machine for sewing a raw edge of cloth, it is necessary to always keep a margin of cloth of a predetermined dimension between the raw edge and the line along the cloth penetrated by the sewing needle. Consequently, it is necessary to neatly cut the raw edge of the cloth immediately before sewing the cloth. For this purpose, in the sewing machine, a lower knife is fixedly mounted on a needle plate disposed in front of the position where the sewing needle penetrates the cloth; and an upper knife, which is so provided as to cooperate with the lower knife to perform a scissor-action.
The scissoring action performed between the upper and the lower knife must be stopped in the following cases: where the edge of the cloth is already cut prior to sewing the cloth; in sewing the last portion of a sleeve-like cloth, where the edges of the cloth are already cut because such last portion of the cloth is partially overlapped with an initially-stitched portion of the cloth; and where pin-tuck stitching is performed.
In a conventional knife drive mechanism of a sewing machine disclosed in Japanese Patent Laid-Open No. Sho 54-152550, which was filed by the applicant of the present invention, as shown in FIG. 7, in operation, an upper knife P moves past a needle plate upward only when cutting operation of the cloth is performed, so that the upper knife cooperates with a lower knife to perform scissoring action. On the other hand, when no cutting of the cloth is required, the upper knife P is held stationary in the bed Q. This prior art mechanism includes a lifting arm R, to which the upper knife P is fixed, and a drive arm S which pivots with arm R, whereby upper knife P moves up-and-down. The drive arm S and a drive lever T, which is driven by a main spindle through a crank motion, are connected to each other through a shaft U, to permit the shaft U to be connected to or disconnected from a connecting/disconnecting element V which is provided in the lifting arm R, whereby motion of the drive lever T is transmitted to the lifting arm R when the connecting/disconnecting element V is engaged with the shaft U. In this manner the lifting arm R moves past the machine bed Q upward together with the upper knife P to perform the scissoring action in cooperation with the lower knife; and, when a dial W is switched, the connecting/disconnecting element V is disengaged from the shaft U to permit the upper knife P to enter the bed Q where the knife P is stationarily received.
It is true that the conventional knife drive mechanism described above is advantageous in that: the upper knife moves past the needle plate upward only during cutting of the cloth; and the upper knife is held under the needle plate when no cutting of the cloth is required. However, in order to stop the scissoring action of the upper and the lower knives, it is necessary to stop the sewing machine, thereby permitting the shaft (on which the upper knife is mounted, the shaft being connected with the drive mechanism) to be axially moved, which interrupts the stitching operation of the machine.
Another conventional knife drive mechanism is disadvantageous in that it is too complicated in construction (see Japanese Utility Model Laid-Open No. Sho 51-16547).
Further, in the prior art, after the scissoring action is stopped, when a raw edge of the cloth extends from the needle plate, there is the danger that the raw edge will get caught by the upper looper to cause the breakage of the upper looper, a sewing needle, a lower looper or the like. Even when the scissoring action of the upper and the lower knives is stopped, in some cases, the upper knife continues its up-and-down motion spaced apart from the lower knife. Because of this, it is apparent that a need exists for preventing the raw edge of the cloth from extending from the throat plate.